Bug 1061450 - Use customized dialog for delete APN waring

https://bugzilla.mozilla.org/show_bug.cgi?id=1061450
It seems this code is redundant : 
```
warningDialog.removeEventListener('click', onclick);
warningDialog.hidden = true;
```

can we integrate these codes into one ? 

```
warningDialog.addEventListener('click', function onclick(event) {
  warningDialog.removeEventListener('click', onclick);
  warningDialog.hidden = true;
  if (event.target === warningDialogOkBtn) {
    resolve(true);
  } else {
    resolve(false);
  }
});
```